The Action Camera L-Bracket from 9.Solutions can be used in a variety of configurations to adjust your camera mounting angle by 90°.
It includes a 1/4"-20 thumbscrew for attaching it to support devices and other mounting accessories, which slots into one of the six holes running the length of the bracket. For attaching camera attachment, the L-bracket features a Quick Mount Receiver than accepts any of 9.Solutions' separately-available Quick Mounts, including 3-prong GoPro and 1/4"-20 screw options.
For added convenience, 9.Solutions thumbscrews have the same size head as the GoPro thumbscrew, so any GoPro wrench can be used tighten them.